To me it's effectively demonstrating you have a means to pay for the fuel, much like checking your wallet before you pick the pump up. By starting the transaction from your phone first it's akin to the cameras on the forecourt recording your registration in case you 'forget to pay' and drive off, if you were going into the shop to pay.
If the app isn't working, or your phone account details play up, it's giving you the choice to abort before you have taken any fuel if you don't have an alternative means to pay with you.
I can see the app mainly being useful to me if I've forgotten my wallet but have my phone (often happens!) as at least I won't be left with no fuel. At this time of year with the amount of people doing a week's food shopping (our local Shell is a mini Waitrose) the option to not have to queue up for ages just to buy fuel has its appeal too 
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===SOLD===2002 Audi S8 D2 Final Edition (yes, really) in Avus Silver with purple extended leather===SOLD===
2011 S5 Sportback in Phantom Black with black Super Sports leather, 9x20s, tech pack high, adaptive xenon plus, intelligent key, memory seats pack, sunroof, B&O, Audi Drive Select & quattro Sports Diff, DAB, parking system plus
2015 VW Golf GTI Performance Pack in Carbon Grey with black Vienna leather, tech pack (Discovery Pro nav & Dynaudio), DCC, factory towbar and retrofit RVC
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